1. Field of the Invention
The invention relates to electronic devices, especially magnetic random access memories, and more specifically to magnetic random access memory arrays compensated for write current shunting.
2. Description of the Related Art
Magnetic random access memory (MRAM) typically is made up of memory cells that store information based on a direction of magnetization of a ferromagnetic region. MRAM cells can hold stored information for long periods of time, and are thus nonvolatile.
MRAM memory generally takes the form of an array of intersecting, electrically-conductive column and row lines formed on a substrate. An MRAM memory cell is located at each intersection of the array.
Various types of MRAM memory cells are known in the prior art. U.S. Pat. No. 5,640,343, issued Jun. 17, 1997 to Gallagher et al., discloses an MRAM device that utilizes an array of Magnetic Tunnel Junction (MTJ) memory cells. Each MTJ MRAM memory cell includes a magnetic tunnel junction. As shown in FIG. 7, a magnetic tunnel junction 70 includes three functional layers: a free ferromagnetic layer 72, a tunnel barrier 74, and a fixed or pinned ferromagnetic layer 76. The magnetization direction of pinned layer 76 is fixed by exchange coupling with an antiferromagnetic material. The magnetization direction of the free ferromagnetic layer 72, in the absence of an applied field, is oriented along either a positive or a negative axis, as indicated by the double-headed arrow.
Conductance of the MTJ memory cell is determined by the alignment of the magnetization in the fixed and free ferromagnetic layers. Accordingly, binary information can be stored in each memory cell based on the relative alignment of the magnetization direction in each layer. When the alignment is parallel (both magnetizations oriented in the same direction) the MTJ has a lower magnetoresistance, representing a logic 0, for example. When the alignment is antiparallel (the magnetizations oriented in opposite directions), the resistance of the MTJ is higher, representing a logic 1. The magnetoresistance is the result of spin-polarized tunneling of conduction electrons between the ferromagnetic layers. Measuring the magnetoresistance indicates the alignment, and thus the logical state, of the cell.
Changing the orientation of the magnetization of the free ferromagnetic layer changes the logic state of the cell. The magnetization of the free layer will rotate in the presence of an applied magnetic field. The magnetic field is applied by way of an electric current flowing in the conductive word and bit lines. A current in both lines generates a magnetic field of sufficient strength to change the orientation state of the memory cell. The current of at least one of the lines has to be reversible to write the two different magnetic states of the cell.